[PATCH] D36772: Unmerge GEPs to reduce register pressure on IndirectBr edges.

[Hiroshi Yamauchi via Phabricator via llvm-commits]

yamauchi added inline comments.


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Comment at: lib/CodeGen/CodeGenPrepare.cpp:6264
+    UGEPI->setOperand(1, NewUGEPIIdx);
+  }
+  // After unmerging, verify that GEPIOp is actually only used in SrcBlock (not
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hfinkel wrote:
> You'll also need to make sure that this GEP is not marked as inbounds if GEPI was not.
> 
>   if (!GEPI->isInBounds()) {
>     UGEPI->setIsInBounds(false);
>   }
> 
> because otherwise the result of GEP could be not-in-bounds resulting in UB if that's used as the input to an inbounds UGEPI.
I'm not very familiar with how inbounds works.

Is an inbounds GEP UB if it takes a non-inbounds GEP as its operand (regardless of whether the index/offset is actually in bounds or not)?

For example,

Before:

%GEPIOp = ...
%GEPI = gep %GEPIOp 2
%UGEPI = gep inbounds %GEPIOp 1

After:

%GEPIOp = ...
%GEPI = gep %GEPIOp 2
%UGEPI = gep inbounds %GEPI -1

Suppose "gep %GEPIOp 2" is not in bounds and "gep inbounds %GEPIOp 1" is in bounds. Both aren't UB.

"gep inbounds %GEPI -1" is UB just because it takes (non-inbounds) "gep %GEPIOp 2" as an operand, even though the offset/index of "gep inbounds %GEPI -1" is actually in bounds?



https://reviews.llvm.org/D36772